Since an oxide superconductor, such as a Y-based oxide superconductor, has a high critical temperature, liquid nitrogen rich in resources can be utilized without use of liquid helium or other substances having a high cost, so that there is a possibility that the range of application of the oxide superconductor can be greatly expanded.
In order for the oxide superconductor to be put to practical use, it is important to enhance the critical current density at a service temperature. In recent years, MPMG process (Melt-Powdering-Melt-Growth; see "Proceedings of ISS 89 Springer-Verlag", 1190, p. 285) (U.S. patent application Ser. No.07/606207) or other processes have enabled the oxide superconductor to be molded into a bulk form and, at the same time, a critical current density on a practical level to be attained. This has led to studies on the application of the oxide superconductor to bearings, flywheels, etc.
The MPMG process comprises melting, for example, a LnBaCuO-based raw material to form a mixed phase comprising an oxide of Ln and a liquid phase, rapidly cooling the mixed phase for solidification, pulverizing the resultant solid, forming a powder, heating the powder to bring it to a partially-molten material comprising Ln.sub.2 BaCuO.sub.5 (hereinafter referred to as "211 phase") and a liquid phase and gradually cooling the partially-molten material under predetermined conditions. According to this process, the 211 phase, which is a LnBaCuO-based non-superconducting phase, can be dispersed in LnBa.sub.2 Cu.sub.3 O.sub.y (hereinafter referred to as "123 phase"). The presence of the 211 phase suppresses the movement of the magnetic flux, which enables a high critical current density to be attained even in a high magnetic field. Such a non-superconducting phase is called "pinning center of magnetic flux".
On the other hand, there is no need to limit the pinning center of magnetic flux to the 211 phase only, and there is a possibility that all heterogeneous portions act as the pinning center. However, when the heterogeneous portions deteriorate superconducting properties of a matrix or form a grain boundary or the like to prevent the flow of a superconducting current, the critical current density is unfavorably lowered. For this reason, the heterogeneous portions capable of functioning as an effective pinning center are limited. Further, it is necessary for the above-described pinning center to be finely and homogeneously dispersed.
In recent years, it has been centered out that BsSnO.sub.3 does not deteriorate the properties of the matrix and is finely dispersed within a crystal to effectively function as a pinning center (see "Japanese Journal of Applied Physics", vol. 29, 1990. 1999).
Under the above-described current circumstances, an object of the present invention is to provide an oxide superconductor having a higher critical current density by virtue of a finer dispersion of the pinning center obtained in the prior art and a-production process that can stably provide the superconductor.